Out-of-bounds recovery circuit

ABSTRACT

Out-of-bounds recovery circuits configured to detect an out-of-bounds violation in an electronic device, and cause the electronic device to transition to a predetermined safe state when an out-of-bounds violation is detected. The out-of-bounds recovery circuits include detection logic configured to detect that an out-of-bounds violation has occurred when a processing element of the electronic device has fetched an instruction from an unallowable memory address range for the current operating state of the electronic device; and transition logic configured to cause the electronic device to transition to a predetermined safe state when an out-of-bounds violation has been detected by the detection logic.

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY

This application is a continuation under 35 U.S.C. 120 of copendingapplication Ser. No. 17/028,253 filed Sep. 22, 2020, now U.S. Pat. No.11,030,039, which is a continuation of prior application Ser. No.15/784,746 filed Oct. 16, 2017, now U.S. Pat. No. 10,817,367, whichclaims foreign priority under 35 U.S.C. 119 from United KingdomApplication No. 1617530.9 filed Oct. 14, 2016.

BACKGROUND

An electronic device comprising a processing element for processingcomputer executable instructions to control operation of the device cantypically operate in one of one or more operating states. An operatingstate may be defined by the program or application that is beingexecuted on the device or any other parameter or feature of the device.For example, a simple electronic device may operate in either a bootoperating state where the processing element is executing a bootprocess, or a normal operating state where the processing element isexecuting an application or program.

Each operating state is typically associated with one or more blocks ofmemory which can be accessed while the electronic device is operating inthat operating state. The memory block(s) associated with a particularoperating state may be used to store executable instructions and/or datathat allow the electronic device to operating in the particular state.For example, in the simple electronic device described above, the bootoperating state may be associated with one or more blocks of BOOT readonly memory (ROM) which are used to store executable instructions and/ordata associated with the boot process; and the normal operating statemay be associated with one or more blocks of random access memory (RAM)which are used to store executable instructions and/or data associatedwith the application or program.

If an electronic device is operating in a particular operating state andaccesses memory that is not associated with the particular operatingstate then there has been an out-of-bounds memory access violation,which may be referred to herein as an out-of-bounds violation. Forexample, if the simple electronic device described above accesses theone or more blocks of RAM associated with the normal operating stateduring the boot process then there has been an out-of-bounds violation.Similarly, if the electronic device accesses the BOOT ROM after the bootprocess has completed then there has been an out-of-bounds violation.

An out-of-bounds violation may cause the electronic device to behaveunpredictably since the accessed memory location does not contain knownor expected data or content.

The embodiments described below are provided by way of example only andare not limiting of implementations which solve any or all of thedisadvantages of known methods and systems for addressing out-of-boundsviolations.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

Described herein are out-of-bounds recovery circuits configured todetect an out-of-bounds violation in an electronic device, and cause theelectronic device to transition to a predetermined safe state when anout-of-bounds violation is detected. The out-of-bounds recovery circuitsinclude detection logic configured to detect that an out-of-boundsviolation has occurred when the processing element has fetched aninstruction from an unallowable memory address for the current operatingstate of the electronic device; and transition logic configured to causethe electronic device to transition to a safe state when anout-of-bounds violation has been detected by the detection logic.

A first aspect provides an out-of-bounds recovery circuit for anelectronic device, the out-of-bounds recovery circuit comprising:detection logic configured to: monitor one or more control and/or datasignals of the electronic device; and detect an out-of-bounds violationin the electronic device, when the detection logic determines, based onthe one or more control and/or data signals of the electronic device, aprocessing element of the electronic device has fetched an instructionfrom a non-allowable memory address for a current operating state of theelectronic device; and transition logic configured to, in response tothe detection logic detecting an out-of-bounds violation, cause theelectronic device to transition to a predetermined safe state.

A second aspect provides a method of recovering an electronic devicefrom an out-of-bounds violation, the method comprising: monitoring, byan out-of-bounds recovery circuit, one or more control and/or datasignals of the electronic device; detecting, by the out-of-boundsrecovery circuit, an out-of-bounds violation in the electronic devicewhen it is determined, from the one or more control and/or data signalsof the electronic device, that a processing element of the electronicdevice has fetched an instruction from a non-allowable memory addressfor a current operating state of the electronic device; and in responseto detecting an out-of-bounds violation, causing, by the out-of-boundsrecovery circuit, the electronic device to transition to a predeterminedsafe state.

A third aspect provides an electronic device comprising a processingelement and the out-of-bounds recovery circuit of the first aspect.

The out-of-bounds recovery circuit may be embodied in hardware on anintegrated circuit. There may be provided a method of manufacturing, atan integrated circuit manufacturing system, an out-of-bounds recoverycircuit. There may be provided an integrated circuit definition datasetthat, when processed in an integrated circuit manufacturing system,configures the system to manufacture an out-of-bounds recovery circuit.There may be provided a non-transitory computer readable storage mediumhaving stored thereon a computer readable description of an integratedcircuit that, when processed, causes a layout processing system togenerate a circuit layout description used in an integrated circuitmanufacturing system to manufacture an out-of-bounds recovery circuit.

There may be provided an integrated circuit manufacturing systemcomprising: a non-transitory computer readable storage medium havingstored thereon a computer readable integrated circuit description thatdescribes the out-of-bounds recovery circuit; a layout processing systemconfigured to process the integrated circuit description so as togenerate a circuit layout description of an integrated circuit embodyingthe out-of-bounds recovery circuit; and an integrated circuit generationsystem configured to manufacture the out-of-bounds recovery circuitaccording to the circuit layout description.

There may be provided computer program code for performing a method asdescribed herein. There may be provided non-transitory computer readablestorage medium having stored thereon computer readable instructionsthat, when executed at a computer system, cause the computer system toperform the method as described herein.

The above features may be combined as appropriate, as would be apparentto a skilled person, and may be combined with any of the aspects of theexamples described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be described in detail with reference to theaccompanying drawings, in which:

FIG. 1 is a block diagram of an example electronic system comprising aprocessing element and an out-of-bounds recovery circuit;

FIG. 2 is a flow diagram of an example method for detecting, andrecovering from, an out-of-bounds violation using the out-of-boundsrecovery circuit of FIG. 1;

FIG. 3 is a block diagram of an example out-of-bounds recovery circuit;

FIG. 4 is a flow diagram of an example method for detecting, andrecovering from, an out-of-bounds violation using the out-of-boundsrecovery circuit of FIG. 3; and

FIG. 5 is a block diagram of an example integrated circuit manufacturingsystem.

The accompanying drawings illustrate various examples. The skilledperson will appreciate that the illustrated element boundaries (e.g.,boxes, groups of boxes, or other shapes) in the drawings represent oneexample of the boundaries. It may be that in some examples, one elementmay be designed as multiple elements or that multiple elements may bedesigned as one element. Common reference numerals are used throughoutthe figures, where appropriate, to indicate similar features.

DETAILED DESCRIPTION

The following description is presented by way of example to enable aperson skilled in the art to make and use the invention. The presentinvention is not limited to the embodiments described herein and variousmodifications to the disclosed embodiments will be apparent to thoseskilled in the art.

Embodiments will now be described by way of example only.

As described above, out-of-bounds violations can cause the electronicdevice to behave unpredictably. In addition, some out-of-boundsviolations can also pose a security threat to the electronic device. Forexample, if a hacker discovers that a processing element of theelectronic device will/can fetch (and execute) an instruction from anout-of-bounds address the hacker could place malicious code at theout-of-bounds memory address to cause a rogue program to execute on theelectronic device.

Such out-of-bounds violations may be caused by the software running onthe device (e.g. an error or bug in the software, or, malicious codeloaded onto the device by, for example, a hacker), or, by an error orbug in the hardware. For example, there may be an error or bug in thehardware that causes the hardware to incorrectly generate the address ofthe next instruction to be fetched from memory.

A method for identifying bugs or errors in a hardware design for anelectronic device that will cause an out-of-bounds violation aredescribed in the Applicant's co-pending patent application filed at theUK Intellectual Property office on the same day as the presentapplication and entitled “DETECTING OUT-OF-BOUNDS VIOLATIONS IN AHARDWARE DESIGN USING FORMAL”. However, while such methods can be usedto ensure that there is not an error in the hardware of the electronicdevice that will cause an out-of-bounds violation, such methods do notprotect the electronic device from an error or bug in the software orfirmware run on the electronic device that causes an out-of-boundsviolation in the electronic device. In other words, such a method doesnot protect the electronic device from security violations caused by thesoftware or firmware itself.

Traditionally security has been considered a problem that is solved viasoftware. For example, security of an electronic device is oftenimplemented by anti-virus, anti-spyware, firewall and/or encryptionsoftware running on the electronic device (or another connected device).However, no matter how much care is exercised in developing andimplementing the security measures some level of compromise may occurresulting in a security violation. Accordingly, a robust and resilientelectronic device is one that not only attempts to prevent securityviolations, but can detect and isolate security violations and drive thesystem into a predetermined good state so that the electronic device cancontinue to operate after a security violation. For example, if there isan out-of-bounds violation caused by a Trojan (i.e. malicious code) inthe firmware running on an electronic device a resilient electronicdevice is one that can detect the out-of-bounds violation and restorethe electronic device to a predetermined good state.

Accordingly described herein are out-of-bounds recovery circuitsconfigured to detect an out-of-bounds violation by an electronic device,and cause the electronic device to transition to a predetermined safestate when an out-of-bounds violation is detected

The out-of-bounds recovery circuits include detection logic configuredto detect an out-of-bounds violation by the electronic device when theprocessing element of the electronic device has fetched an instructionfrom an unallowable memory address for the current operating state ofthe electronic device; and transition logic configured to cause theelectronic device to transition to a safe state when an out-of-boundsviolation has been detected by the detection logic.

Detecting out-of-bounds violations in hardware, rather than software,allows the detection to be performed more efficiently. It also allowsout-of-bounds violations caused by an error in the firmware (especiallyboot firmware) to be detected. In particular, security software, such asanti-virus programs, can generally only be invoked on an electronicdevice after the electronic device has completed its boot sequence thusany error or bug in the boot firmware may not be identified by any suchsecurity software that is invoked after the boot sequence.

Furthermore, the hardware cannot be modified after manufacture so thehardware (if designed correctly) is not subject to hacker attacks andthus will always be able to detect an out-of-bounds violation and putthe electronic device in a state which enables the electronic device torecover from the violation. Thus an electronic device with anout-of-bounds recovery circuit will be more robust to security attacks.

Reference is now made to FIG. 1 which illustrates an example electronicdevice 100 with an out-of-bounds recovery circuit 104 for detectingout-of-bounds violations and transitioning the electronic device 100 toa predetermined safe state when an out-of-bounds violation is detected.

An electronic device 100 is any kind of device, machine, or dedicatedcircuit, or collection or portion therefore comprising one or moreprocessing elements 102 that can execute computer-readable instructionsstored in memory 106. Those skilled in the art will realize that suchprocessing capabilities are incorporated into many different devices andtherefore the term ‘electronic device’ includes set top boxes, mediaplayers, digital radios, PCs, servers, mobile telephones, personaldigital assistants and many other devices. Although the electronicdevice 100 of FIG. 1 comprises a single processing element 102, otherelectronic devices may comprise more than one processing element.

As is known to those of skill in the art, a processing element 102fetches instructions from a program stored in memory 106, decodes thefetched instructions, executes the decoded instructions, andstores/commits the results of the executed instructions in memory and/orregisters. The processing element 102 may be any kind of device, machineor dedicated circuit, or collection or portion thereof, with processingcapability such that it can execute instructions. The processing element102 may be a pipelined or un-pipelined processor, a single-threaded ormulti-threaded processor, an in-order or out-of-order processor or anycombination thereof.

A processing element may be any kind of general purpose or dedicatedprocessor, such as a CPU, GPU, System-on-chip, state machine, mediaprocessor, an application-specific integrated circuit (ASIC), aprogrammable logic array, a field-programmable gate array (FPGA),physics processing units (PPUs), radio processing units (RPUs), digitalsignal processors (DSPs), general purpose processors (e.g. a generalpurpose GPU), microprocessors, any processing unit which is designed toaccelerate tasks outside of a CPU, etc.

The memory 106 may be any technology suitable for storing computerreadable instructions such as RAM, ROM, EPROM, EEPROM, flash memory,other memory technology, or any combination thereof. Although the memory106 is shown within the electronic device 100 it will be appreciatedthat the memory 106 may be distributed or located remotely from theelectronic device 100 and accessed by the electronic device 100 via anetwork or other communication link.

The electronic device 100 of FIG. 1 can operate in one or more operatingstates. Each operating state may be defined by the program orapplication that is being executed by the electronic device 100 or anyother parameter or feature of the electronic device 100. The electronicdevice 100 may have one operating state or more than one operatingstate. For example, a simple electronic device, such as a powermanagement unit (PMU) may be operable in one of two operating states: aboot operating state where, for example, the electronic device 100 isimplementing a boot process; and a normal operating state where theelectronic device 100 is executing one or more programs or applications.The electronic device 100 may initially operate in the boot operatingstate, and transition to the normal operating state after the bootprocess is complete.

Each operating state is associated with one or more blocks of memory 106that the electronic device 100 is permitted to access while operating inthat operating state. Such blocks of memory are referred to herein asallowable memory blocks for the particular operating state. The memoryblocks that are not part of the allowable memory blocks for a particularoperating state are referred to herein as the unallowable memory blocksfor the particular operating state. Each block of memory is defined andaccessible by a memory address range. The address range defining anallowable memory block will be referred to herein as an allowable memoryaddress range and the address range defining an unallowable memory blockwill be referred to herein as an unallowable memory address range.

The allowable blocks of memory may be used to store executableinstructions and/or data that allow the electronic device 100 to operatein that particular operating state. For example, the boot operatingstate may be associated with a special type of read only memory (ROM)referred to as BOOT ROM, whereas the normal operating state may beassociated with another type of memory such as random access memory(RAM).

It is expected that while the electronic device 100 is operating in aparticular state that the electronic device 100 will only access theallowable memory blocks for that operating state. However, as describedabove, there may be an error or bug in the software or firmware executedby the electronic device 100 that causes the electronic device 100 toaccess an unallowable memory block which is referred to herein as anout-of-bounds violation. One of the most significant out-of-boundsviolations is loading an instruction from an unallowable memory block asit may allow a rogue program to execute on the electronic device 100.

The out-of-bounds recovery circuit 104 is configured to detect anout-of-bounds violation in the electronic device 100 and aid theelectronic device 100 in recovering from the violation by causing, orforcing, the electronic device 100 to transition to a predetermined goodstate. The out-of-bounds recovery circuit 104 comprises detection logic108 and transition logic 110.

The detection logic 108 is configured to monitor one or more controland/or data signals of the electronic device 100 to detect when anout-of-bounds violation has occurred. In particular, the detection logic108 is configured to identify, from the one or more control and/or datasignals, the current operating state of the electronic device 100, anddetect an out-of-bounds violation when the detection logic 108 detects,from the one or more control and/or data signals, that the processingelement 102 has fetched an instruction from an unallowable memoryaddress for the current operating state

The specific control signals and/or data signals that are monitored, andthe status thereof indicating a particular operating state will varydepending on the configuration of the electronic device 100. Forexample, where the electronic device is a PMU that can operate in a bootoperating state or a normal operating state the detection logic 108 maybe configured to monitor a signal (e.g. a power_on_complete signal) thatindicates when the boot process is complete to determine whether the PMUis in the boot operating state or the normal operating state. It will beevident that this is an example only and other control signals and/ordata signals may be monitored to determine the current operating stateof the electronic device 100.

Similarly the specific control and/or data signals that are monitored,and the status thereof indicating an instruction has been fetched froman unallowable memory address will vary depending on the configurationof the electronic device 100. For example, the detection logic 108 maybe configured to monitor a signal which indicates the current value ofthe program counter of the processing element 102 and compare thecurrent value of the program counter to an allowable address rangeand/or an unallowable address range to determine if the processing unit102 has fetched an instruction from an unallowable memory address. Wherethe electronic device is a PMU that comprises an interpreter that actsas the processing element 102 to decode and execute instructions thesignal indicating the current value of the program counter may be, forexample, an address signal (e.g. pmu_addr).

The transition logic 110 is configured to (in response to the detectionlogic 108 detecting an out-of-bounds violation) cause the electronicdevice 100 to transition to a predetermined safe state. A predeterminedsafe state is a state that the out-of-bounds recovery circuit 104 knowshas not been compromised and allows the electronic device 100 tocontinue operating. The specific predetermined safe state or states willdepend on the specific electronic device 100 being monitored. Forexample, in some cases the transition logic 110 may be configured tocause the electronic device 100 to transition to a predetermined stateby causing the processing element 102 of the electronic device 100 totransition into an idle state or a post-reset state.

The transition logic 110 may be configured to cause the processingelement 102 to transition to the predetermined safe state in any knownmanner. In some cases the processing element 102 of the electronicdevice 100 may comprise a state tracking mechanism for tracking thestate of the processing element 102. For example, the state trackingmechanism may be a state machine which uses (signal, value) pairs tomove the state machine into different states. In these cases thetransition logic 110 may be configured to cause the processing element102 to transition to a particular state by setting a particular signalto a predetermined value.

For example, where the electronic device 100 is a PMU the processingelement (e.g. interpreter) 102 may comprise a state machine which ismoved into the PMU's next assigned state each cycle. However, if anout-of-bounds violation is detected, the transition logic 110 may beconfigured to override the assigned state by causing the state machineto move into, for example, an IDLE state where the processing element(e.g. interpreter) 102 awaits further external interaction.

In other cases the transition logic 110 may be configured to cause theelectronic device 100 to transition to a predetermined safe state bycausing an interrupt to invoke a particular exception handler. In yetother cases the transition logic 110 may be configured to cause theelectronic device 100 to transition to a predetermined safe state byforcing the processing element 103 of the electronic device 100 into apredetermined state and causing an interrupt to invoke a particularexception handler.

Although the out-of-bounds recovery circuit 104 is shown in FIG. 1 asbeing within (or part of) the electronic device 100 it will beappreciated that the out-of-bounds recovery circuit 104 may be externalto, but connected or coupled to, the electronic device 100.

An example implementation of the out-of-bounds recovery circuit 104 willbe described below with reference to FIG. 3.

In some cases, the out-of-bounds recovery circuits 104 and 304 describedherein may be integrated into the design of the electronic device 100 sothat when the electronic device 100 is manufactured the out-of-boundsrecovery circuit 104 forms part of the electronic device 100. Forexample, if the electronic device 100 is implemented in a hardwaredesign and the out-of-bounds recovery circuit 104 is implemented in ahardware design, the hardware design for the out-of-bounds recoverycircuit 104 or 304 may be integrated into the hardware design for theelectronic device 100, or, the hardware design for the out-of-boundsrecovery circuit 104 or 304 may be bound to the hardware design for theelectronic device 100.

The term “hardware design”, which may also be referred to as anintegrated circuit definition dataset, is used herein to refer to adescription of the structure and function of an integrated circuit whichwhen processed at an integrated circuit manufacturing system causes theintegrated circuit manufacturing system to generate an integratedcircuit described by the hardware design. For example, as described inmore detail below with respect to FIG. 5, when a hardware design isprocessed at an integrated circuit manufacturing system the integratedcircuit manufacturing system may generate the integrated circuit bysynthesizing the hardware design into silicon, or, by loadingconfiguration data into a field-programmable gate array (FPGA).

A hardware design may be implemented in a high level hardwaredescription language (HDL), such as, but not limited to, a registertransfer level (RTL) language. Examples of register transfer levellanguages include, but are not limited to, VHDL (VHS IC HardwareDescription Language) and Verilog. It will be evident to a person ofskill in the art that other high level hardware description languagesmay be used such as proprietary high level hardware descriptionlanguages.

Reference is now made to FIG. 2 which illustrates an example method 200for detecting, and recovering from, an out-of-bounds violation in anelectronic device 100 which may be implemented by the out-of-boundsrecovery circuit 104 of FIG. 1.

The method 200 begins at block 202 where the detection logic 108 of theout-of-bounds recovery circuit 104 monitors one or more relevant controland/or data signals of the electronic device 100. Monitoring the one ormore relevant controls signals may comprise sampling some or all of therelevant control signals over one or more clock cycles. As describedabove, the relevant control signals that are monitored will depend onthe electronic device 100 being monitored.

At block 204, the detection logic 108 determines, based on the status ofthe monitored control and/or data signal(s), whether an out-of-boundsviolation has occurred in the electronic device. In particular, thedetection logic 108 determines based on the status of the monitoredcontrol and/or data signals(s) whether the processing element 102 of theelectronic device 100 has fetched an instruction from an unallowablememory address for the current operating state of the electronic device100.

As described above, in some cases the detection logic 108 may beconfigured to identify the current operating state from the monitoredcontrol and/or data signal(s) and then determine if there has been anout-of-bounds violation by comparing the program counter of theprocessing element 102 to the allowable and/or unallowable memoryaddress ranges for the current operating state to determine if theelectronic device 100 has fetched an instruction from an unallowablememory address for the current operating state.

If the detection logic 108 determines that the electronic device 100 hasfetched an instruction from an unallowable memory address (and thus anout-of-bounds violation has occurred) the method 200 proceeds to block206. If, however, the detection logic 108 does not determine that theelectronic device 100 has fetched an instruction from an unallowablememory address, the method 200 proceeds back to block 202 where thedetection logic 108 continues to monitor the one or more relevantcontrol signals.

At block 206, once the detection logic 108 has detected a fetch of aninstruction from an unallowable memory address (and thus anout-of-bounds violation has occurred) the transition logic 110 causesthe electronic device 100 to transition to a predetermined safe state.As described above, a predetermined safe state is any state of theelectronic device 100 in which the electronic device 100 should not beable to make a transition to an unsafe state. In particular, once theelectronic device 100 is in a safe state it is expected that theelectronic device 100 will transition to states that are expected arealigned with the normal functioning of the electronic device 100. Forexample, the transition logic 110 may cause the electronic device totransition to a predetermined safe state by causing the processingelement 102 of the electronic device 100 to transition to an IDLE state.The IDLE state is considered a safe state because once the processingelement 102 is in the IDLE state the processing element 102 can onlymake transitions to well defined states that are expected in the normalexecution of the electronic device 100. Once the transition logic 110has caused the electronic device 100 to transition to a predeterminedsafe state the method 200 may end or the method 200 may return to block202 where the detection logic 108 continues to monitor the relevantcontrol signals.

Reference is now made to FIG. 3 which illustrates an exampleimplementation of an out-of-bounds recovery circuit 304, which may beused as the out-of-bounds recovery circuit 104 of FIG. 1 for detecting,and recovering from an out-of-bounds violation in an electronic device100. The out-of-bounds recovery circuit 304 of FIG. 3 detects anout-of-bounds violation in the electronic device 100 when the processingelement 102 of the electronic device 100 fetches an instruction from anunallowable memory address.

The out-of-bounds recovery circuit 304 comprises detection logic 308 fordetecting when an out-of-bounds violation has occurred in the electronicdevice 100 and transition logic 310 for causing the electronic device100 to transition to a predetermined safe state.

The detection logic 308 is configured to detect that an out-of-boundsviolation has occurred in the electronic device 100 when the processingelement 102 of the electronic device 100 has fetched an instruction froman unallowable memory address. The detection logic 308 comprisesout-of-bounds fetch detection logic 320 which monitors one or morecontrol and/or data signals of the electronic device 100 that indicatethe current operating state of the electronic device 100 to identify thecurrent operating state of the electronic device.

For example, where the electronic device 100 is a PMU that initiallyoperates in a boot operating state where the electronic device isimplementing a boot process, and once the boot process is completeoperates in a normal operating state where the electronic device 100 isexecuting one or more programs or applications, the detection logic 308may be configured to monitor a signal (e.g. a power_on_complete signal)that indicates whether or not the electronic device 100 has completedthe boot process. If the signal indicates that the electronic device 100has not completed the boot process then the detection logic identifiesthat the PMU is in the boot operating state, and if the electronicdevice 100 has completed the boot process then the out-of-bounds fetchdetection logic 320 identifies that the PMU is in the normal operatingstate.

In some cases, as shown in FIG. 3, the out-of-bounds fetch detectionlogic 320 may be configured to store in a storage unit, such as aregister or flag, information indicating the identified currentoperating state of the electronic device 100. For example, in the PMUexample, the detection logic 308 may comprise an operating stateregister 322 and the out-of-bounds fetch detection logic 320 may beconfigured to set the operating state register 322 (e.g. to a “1” or“true” value) when the out-of-bounds fetch detection logic 320identifies that the electronic device 100 is currently operating in theboot operating state, and clear the operating state register 322 (e.g.to a “0” or false value) when out-of-bounds fetch detection logic 320identifies that the electronic device 100 is currently operating in thenormal operating state. The information stored in the storage unit (suchas operating state register 322) may then be used to determine thecurrent operating state for the purpose of identifying the allowable andunallowable memory address range(s). In other cases, the out-of-boundsfetch detection logic may use the signal(s) indicating the currentoperating state directly for the purpose of identifying the allowableand unallowable memory address ranges.

The out-of-bounds fetch detection logic 320 also monitors one or morecontrol signals that indicate the current value of the processingelement's program counter to determine the memory address of a fetchedinstruction. As is known to those of skill in the art, the programcounter (PC) of a processing element tells the processing element wherein memory to fetch the next instruction for execution from. Inparticular, the PC typically indicates an address in memory from whichthe next instruction should be fetched. Then when the processing elementis in the fetch state the processing element fetches an instruction fromthe address in memory pointed to by the PC.

The out-of-bounds fetch detection logic 320 is configured to compare thevalue of the program counter to allowable and/or unallowable memoryaddress ranges associated with the current operating state to determineif the processing element 102 of the electronic device 100 has fetchedan instruction from an unallowable memory address. Generally if thevalue of the program counter falls within an allowable memory addressrange for the current operating state then the processing element 102has fetched an instruction from an allowable memory address range, and,conversely if the value of the program counter falls outside theallowable memory address range(s) for the current operating state (i.e.the value of the program counter falls within an unallowable memoryaddress range for the current operating state) then the processingelement has fetched an instruction from an unallowable memory addressrange and thus an out-of-bounds violation has occurred.

For example, if the electronic device has a first operating state thathas an allowable address range from address A to address B, and a secondoperating state that has an allowable address range from address C toaddress D then the out-of-bounds fetch detection logic 320 may beconfigured to compare the value of the program counter to the allowableaddress range defined by address A and address B, or the allowableaddress range defined by address C and address D based on the currentoperating state of the electronic device 100 (as determined from theoperating state register 322, or as determined directly from the one ormore control and/or data signals indicating the current operating stateof the electronic device 100). If the value of the program counter fallswithin the allowable memory address range associated with the currentoperating state then the processing element 102 has fetched aninstruction from an allowable memory address. In contrast, if the valueof the program counter does not fall within an allowable memory addressrange associated with the current operating state then the processingelement 102 has fetched an instruction from an unallowable memoryaddress and thus there has been an out-of-bounds violation.

In some cases, the allowable address range(s) (and/or unallowableaddress range(s)) associated with each operating state may be hard-codedinto the out-of-bounds fetch detection logic 320. In other cases, thedetection logic 308 may comprise address range generator logic 323 whichis configured to generate the allowable and/or unallowable addressrange(s) associated with each operating state based on one or more inputparameters.

For example, where the electronic device 100 is a PMU that is operablein a boot operating state and a normal operating state and the bootoperating state is associated with BOOT ROM and the normal operatingstate is associated with a block of RAM, and the addresses allocated tothe RAM follow the addresses allocated to the BOOT ROM, the addressrange generator logic 323 may be configured to generate the memoryaddress ranges for the boot operating state and the normal operatingstate based on the base address for the BOOT ROM, the size of the BOOTROM and the size of the block of RAM. The following is example code todetermine the top and bottom of the address ranges for the BOOT ROM andthe relevant block of RAM:

-   -   BOTTOM_ADDR_BOOT_ROM=BOOT_ROM_BASE_ADDRESS;    -   TOP_ADDR_BOOT_ROM=BOOT_ROM_BASE_ADDRESS+BOOT_ROM_SIZE;    -   BOTTOM_ADDR_RAM=BOOT_ROM_BASE_ADDRESS+BOOT_ROM_SIZE+ONE_ADDRESS;    -   TOP_ADDR_RAM=BOOT_ROM_BASE_ADDRESS+BOOT_ROM_SIZE+RAM_SIZE;

If the out-of-bounds fetch detection logic 320 determines that anout-of-bounds violation has occurred then the out-of-bounds fetchdetection logic 320 generates an indication that an out-of-boundsviolation has been detected. In the example of FIG. 3 the out-of-boundsfetch detection logic 320 is configured to set an out-of-boundsviolation detected register 324 to indicate that an out-of-boundsviolation has been detected. However, in other examples theout-of-bounds fetch detection logic 320 may generate a signal thatindicates an out-of-bounds violation has been detected.

The transition logic 310 is configured to (upon determining that thedetection logic 308 has detected that an out-of-bounds violation hasoccurred in the electronic device 100) cause the electronic device 100to transition to a predetermined safe state. In some cases, thetransition logic 310 is configured to determine that the detection logic308 has detected an out-of-bounds violation based on the status of theout-of-bounds violation detected register 324. For example, in somecases the transition logic 310 is configured to determine that thedetection logic 308 has detected an out-of-bounds violation when theout-of-bounds violation detected register 324 is set and to determinethat the detection logic 308 has not detected an out-of-bounds violationotherwise.

Once the transition logic 310 has determined that the detection logic308 has detected an out-of-bounds violation the transition logic 310causes the electronic device 100 to transition to a safe state. In theexample shown in FIG. 3 the transition logic 310 is configured to causethe electronic device 100 to transition to a safe state by putting theprocessing element 102 in a particular state (e.g. IDLE state).

As described above, in some cases the processing element 102 maycomprise a state tracking mechanism for tracking the state of theprocessing element 102. For example, the state tracking mechanism may bea state machine which uses (signal, value) pairs to move the statemachine into different states. In these cases, the transition logic 310may be configured to cause the processing element 102 to transition to aparticular state by setting a particular signal to a predeterminedvalue.

For example, where the electronic device 100 is a PMU the processingelement (e.g. interpreter) 102 may comprise a state machine which ismoved into the PMU's next assigned state each cycle. However, if anout-of-bounds violation is detected, the transition logic 310 may beconfigured to override the assigned state by causing the state machineto move into, for example, an IDLE state where the processing element(e.g. interpreter) 102 awaits further external interaction.

In some cases, causing the processing element 102 to transition to anIDLE state may also include clearing and/or setting one or moreregisters to ensure that the processing element 102 can start processinginstructions from a known point.

It will be evident to a person of skill in the art that this is anexample only and the transition logic 310 may be configured to cause theelectronic device 100 to transition to another predetermined safe stateand/or in another manner. For example, in other cases the transitionlogic 310 may be configured to invoke an interrupt (e.g. by driving aninterrupt line) to cause the processing element to execute a particularexception handler.

The transition logic 310 may also be configured to set the current fetchto invalid (e.g. by setting a flag) so that the processing element doesnot execute the instruction fetched from the out-of-bounds address.

Although, not shown in FIG. 3, it will be evident to a person of skillin the art that the logic blocks (e.g. out-of-bounds fetch detectionlogic 320, address range generator logic 323, and transition logic 310)may be triggered by a clock. For example, one or more of the logicblocks may be triggered by the rising or positive edge of the clock.Furthermore, it will be evident to a person of skill in the art that oneor more of the logic blocks (e.g. out-of-bounds fetch detection logic320, address range generator logic 323, and transition logic 310) may becombined or their functionality may be divided between logic blocks inanother manner.

Reference is now made to FIG. 4 which illustrates an example method 400for detecting, and recovering from, an out-of-bounds violation in anelectronic device 100 which may be implemented by the out-of-boundsrecovery circuit 304 of FIG. 3. The method 400 detects that anout-of-bounds violation has occurred in the electronic device 100 if theprocessing element 102 of the electronic device 100 has fetched aninstruction from an unallowable memory address.

The method 400 begins at block 402 where the out-of-bounds fetchdetection logic 320 monitors one or more control and/or data signals ofthe electronic device 100 that indicate the current operating state ofthe electronic device 100, and one or more control and/or data signalsthat indicate the value of the program counter for the electronicdevice's processing element 102. Monitoring the one or more controland/or data signals may comprise periodically sampling the one or morecontrol and/or data signals. The specific control and/or data signalsthat are monitored are dependent on the configuration of the particularelectronic device 100 being monitored. The method 400 then proceeds toblock 404.

At block 404, the out-of-bounds fetch detection logic 320 identifies,from the monitored control and/or data signal(s), the current operatingstate of the electronic device 100. For example, as described above,where the out-of-bounds fetch detection logic 320 is configured tomonitor a signal (e.g. a power_on _complete signal) that indicateswhether the power on process is complete, the out-of-bounds fetchdetection logic 320 may be configured to identify that the electronicdevice 100 (e.g. PMU) is in a first operating state (e.g. boot operatingstate) when the monitored signal indicates that the power on process isnot complete, and to identify that the electronic device 100 (e.g. PMU)is in a second operating state (e.g. normal operating state) when themonitored signal indicates that the power on process is complete.

As described above, in some cases the out-of-bounds fetch detectionlogic 320 may store the results of the identification in a storagemodule, such as, but not limited to a register (e.g. operating stateregister 322). For example, the out-of-bounds fetch detection logic 320may store a value in the storage module that indicates the results ofthe identification.

Once the out-of-bounds fetch detection logic 320 has identified thecurrent operating state of the electronic device 100 the method 400proceeds to block 406.

At block 406, the out-of-bounds fetch detection logic 320 compares thevalue of the program counter of the processing element 102 of theelectronic device 100, based on the monitored control and/or datasignal(s), to allowable address range(s) and/or unallowable addressrange(s) for the identified operating state to determine if theprocessing element 102 has fetched an instruction from an unallowablememory address.

For example, if the electronic device has a first operating state thathas an allowable address range from address A to address B, and a secondoperating state that has an allowable address range from address C toaddress D, then if the out-of-bounds fetch detection logic 320 hasdetected that the electronic device 100 is operating in the firstoperating state then the out-of-bounds fetch detection logic 320 maycompare the value of the program counter to the allowable address rangedefined by address A and address B, and if the out-of-bounds fetchdetection logic 320 has detected that the electronic device 100 isoperating in the second operating state then the out-of-bounds fetchdetection logic 320 may compare the value of the program counter to theallowable address range defined by address C and address D.

Once the value of the program counter has been compared to the allowableand/or unallowable memory address range(s) associated with the operatingstate identified in block 404 the method 400 proceeds to block 408.

At block 408, the out-of-bounds fetch detection logic 320, based on thecomparison, determines whether the processing element 102 of theelectronic device 100 has fetched an instruction from an unallowablememory address thus indicating an out-of-bounds violation. Inparticular, if the value of the program counter falls within at leastone allowable memory address range associated with the current operatingstate of the electronic device 100 then the processing element 102 hasfetched an instruction from an allowable memory address. In contrast, ifthe value of the program counter does not fall within at least oneallowable memory address range associated with the current operatingstate of the electronic device 100 then the processing element 102 hasfetched an instruction from an unallowable memory address and there hasbeen an out-of-bounds violation.

If the out-of-bounds fetch detection logic 320 has determined that theelectronic device 100 has fetched an instruction from an unallowablememory address (indicating that there has been an out-of-boundsviolation) the method 400 proceeds to block 410. In contrast, if theout-of-bounds fetch detection logic 320 has determined that theelectronic device 100 has fetched an instruction from an allowablememory address the method 400 proceeds back to block 402 where theout-of-bounds fetch detection logic 320 continues to monitor the one ormore control and/or data signals.

At block 410, upon detecting an out-of-bounds violation, theout-of-bounds fetch detection logic 320 generates an indicator that anout-of-bounds violation has been detected. In some cases, theout-of-bounds fetch detection logic 320 may set a register, such as theout-of-bounds violation detected register 324, to indicate that anout-of-bounds violation has been detected. In other cases, theout-of-bounds fetch detection logic 320 may output an out-of-boundsviolation detected signal indicating that an out-of-bounds violation hasbeen detected. Once the out-of-bounds fetch detection logic 320 hasgenerated the indicator that an out-of-bounds violation has beendetected, the method 400 proceeds to block 412.

At block 412, the transition logic 310 receives the indicator generatedby the out-of-bounds fetch detection logic 320 that an out-of-boundsviolation has been detected. For example, the transition logic 310 mayread the value of the out-of-bounds violation detected register 324, orthe transition logic 310 may receive the generated out-of-boundsdetected signal. Once the transition logic 310 has received theindicator that an out-of-bounds violation has been detected, the method400 proceeds to block 414.

At block 414, the transition logic 310 causes the electronic device 100to transition to a predetermined safe state. In some cases, thetransition logic 100 may cause the electronic device 100 to transitionto a predetermined safe state by setting the processing element 102 ofthe electronic device 100 to a predetermined state (e.g. an idle state).In other cases, the transition logic 310 may cause the electronic device100 to transition to a predetermined safe state by invoking an interrupt(e.g. by driving an interrupt line) to cause the processing element 102to execute a particular exception handler. However, it will be evidentto a person of skill in the art that these are examples only and thetransition logic 310 may cause the electronic device 100 to transitionto another predetermined safe state and/or in another manner.

In some cases, as part of transitioning the electronic device 100 to apredetermined safe state, or in addition to transitioning the electronicdevice 100 to a predetermined safe state, the transition logic mayconfigure or set one or more signals and/or parameters of the electronicdevice 100 to ensure safe operation. For example, in some cases thetransition logic 100 may set the current fetch to invalid (e.g. bysetting a flag) so that the processing element 102 does not execute theinstruction fetched from the out-of-bounds memory address.

The following is example code for implementing transition logic 310which is configured to transition a PMU to a predetermined safe stateupon detecting fetch of an instruction by the instruction interpreter(i.e. processor) of the PMU from an out-of-bounds memory address for thecurrent operating state of the PMU by (i) raising an interrupt; (ii)disabling a number of valid signals; and (ii) setting the current stateof the instruction interpreter (i.e. processor of the PMU) to IDLE.

In particular, in this example, out_of _bound is the out-of-boundsviolation detected register 324 which is set to “1” when theout-of-bounds fetch detection logic 320 detects an out-of-boundsviolation; pmu_memory_out_of_ram _irq is a PMU interrupt line which isset to “1” (raising an interrupt) when an out-of-bounds violation isdetected.

The pmu_ad_val signal is a PMU signal that indicates that the programcounter has valid data (i.e. that the address is valid). When theout-of-bounds fetch detection logic 320 does not detect an out-of-boundsviolation the value of this signal is stored in a pmu_safe _ad_valregister otherwise the pmu_safe _ad_val register is set to 0. In otherwords, pmu_safe _ad_val is set to the pmu_ad _val value when there hasnot been an out-of-bounds violation, and otherwise the pmu_safe _ad_valis set to ‘0’. Accordingly, pmu_safe _ad_val indicates whether theprogram counter has valid data AND safe data.

The pmu_maon _ad_val is a PMU signal that controls communication to thePMU always on domain, and the pmu_soc _ad_val is a PMU signal that sendsdata to the network on chip (NoC) fabric. Both of these signals are setto the pmu_safe _ad_val value.

pmu_memory_out_of_ram_irq <= ‘1’ when (out_of_bound = ‘1’) else ‘0’;pmu_safe_ad_val <= pmu_ad_val when (out_of_bound = ‘0’) else ‘0’;pmu_maon_ad_val <= pmu_safe_ad_val pmu_soc_ad val <= pmu_safe_ad_valpmu_state_proc : process (clk, resetn) begin  if (resetn = ‘0’) thenpmu_current_state <= pmu_reset;  elsif (clk′event and clk = ‘1’) then if((out_of_bound = ‘1’)) then pmu_current_state <= pmu_idle;r_ext_inst_address <= (others => ‘0’); external_execution <= ‘0’;timeout_counter_resetn <= ‘0’; power_on_complete_i <= ‘1’; elsepmu_current_state <= pmu_next_state; end if;  end if; end process;The pmu_state _proc is a process that, upon an out-of-bounds violationbeing detected (out_of _bound=‘1’), the current state of the PMU'sinstruction interpreter is set to IDLE (pmu_current _state<=pmu_idle),the external address is set to all zeros (r_ext_int_address<=(others=>‘0’)), external execution (i.e. execution of externalprograms) is disabled (external_execution<=‘0’), the timer counter isreset (timeout_counter _resetn<=‘0’), and the power on complete signalis set to say that the boot sequence has completed (power_on_complete_i<=‘1’). If an out-of-bound violation has not been detectedthen the PMU's instruction interpreter is driven to the next state tocontinue operating as normal.

It will be evident to a person of skill in the art that this is anexample only and the transition logic 310 may be configured totransition an electronic device 100 to a predetermined safe state inother ways and/or using other logic or signals.

Once the transition logic 310 has caused the electronic device 100 totransition to a predetermined safe state the method 400 proceeds back toblock 402 where the out-of-bounds fetch detection logic 320 continues tomonitor the control and/or data signals.

It will be evident to a person of skill in the art that the above methodis an example only and the blocks of the method may be executed inanother order and some of the blocks may be omitted entirely. Forexample, in other examples blocks 410 and 412 may be omitted entirelyand the method may proceed directly from block 408 to block 414.

Block 402 generally corresponds to block 202 of FIG. 2, blocks 404 to410 generally correspond to block 204 of FIG. 2, and blocks 412 and 414generally correspond to block 206 of FIG. 2.

The out-of-bounds recovery circuits of FIGS. 1 and 3 are shown ascomprising a number of functional blocks. This is schematic only and isnot intended to define a strict division between different logicelements of such entities. Each functional block may be provided in anysuitable manner. It is to be understood that intermediate valuesdescribed herein as being formed by an out-of-bounds recovery circuitneed not be physically generated by the out-of-bounds recovery circuitat any point and may merely represent logical values which convenientlydescribe the processing performed by the out-of-bounds recovery circuitbetween its input and output.

The terms computer program code and computer readable instructions asused herein refer to any kind of executable code for processors,including code expressed in a machine language, an interpreted languageor a scripting language. Executable code includes binary code, machinecode, bytecode, code defining an integrated circuit (such as a hardwaredescription language or netlist), and code expressed in a programminglanguage code such as C, Java or OpenCL. Executable code may be, forexample, any kind of software, firmware, script, module or librarywhich, when suitably executed, processed, interpreted, compiled,executed at a virtual machine or other software environment, cause aprocessor of the computer system at which the executable code issupported to perform the tasks specified by the code.

A hardware design, which also may be referred to herein as an integratedcircuit definition data set, describes an integrated circuit. Thehardware design may be implemented in software, such as HDL (hardwaredescription language). Accordingly, there may be provided a computerreadable storage medium having encoded thereon computer readable programcode in the form of an integrated circuit definition dataset (e.g.hardware design) that when processed in an integrated circuitmanufacturing system configures the system to manufacture anout-of-bounds recovery circuit.

An integrated circuit definition dataset may be in the form of computercode, for example as a netlist, code for configuring a programmablechip, as a hardware description language defining an integrated circuitat any level, including as register transfer level (RTL) code, ashigh-level circuit representations such as Verilog or VHDL, and aslow-level circuit representations such as OASIS® and GDSII. Higher levelrepresentations which logically define an integrated circuit (such asRTL) may be processed at a computer system configured for generating amanufacturing definition of an integrated circuit in the context of asoftware environment comprising definitions of circuit elements andrules for combining those elements in order to generate themanufacturing definition of an integrated circuit so defined by therepresentation. As is typically the case with software executing at acomputer system so as to define a machine, one or more intermediate usersteps (e.g. providing commands, variables etc.) may be required in orderfor a computer system configured for generating a manufacturingdefinition of an integrated circuit to execute code defining anintegrated circuit so as to generate the manufacturing definition ofthat integrated circuit.

An example of processing an integrated circuit definition dataset at anintegrated circuit manufacturing system so as to configure the system tomanufacture an out-of-bounds recovery circuit will now be described withrespect to FIG. 5.

FIG. 5 shows an example of an integrated circuit (IC) manufacturingsystem 502 which comprises a layout processing system 504 and anintegrated circuit generation system 506. The IC manufacturing system502 is configured to receive an IC definition dataset (e.g. a definingan out-of-bounds recovery circuit as described in any of the examplesherein), process the IC definition dataset, and generate an IC accordingto the IC definition dataset (which embodies an out-of-bounds recoverycircuit as described in any of the examples herein). The processing ofthe IC definition dataset configures the IC manufacturing system 502 tomanufacture an integrated circuit embodying an out-of-bounds recoverycircuit as described in any of the examples herein.

The layout processing system 504 is configured to receive and processthe IC definition dataset to determine a circuit layout. Methods ofdetermining a circuit layout from an IC definition dataset are known inthe art, and for example may involve synthesising RTL code to determinea gate level representation of a circuit to be generated, e.g. in termsof logical components (e.g. NAND, NOR, AND, OR, MUX and FLIP-FLOPcomponents). A circuit layout can be determined from the gate levelrepresentation of the circuit by determining positional information forthe logical components. This may be done automatically or with userinvolvement in order to optimise the circuit layout. When the layoutprocessing system 504 has determined the circuit layout it may output acircuit layout definition to the IC generation system 506. A circuitlayout definition may be, for example, a circuit layout description.

The IC generation system 506 generates an IC according to the circuitlayout definition, as is known in the art. For example, the ICgeneration system 506 may implement a semiconductor device fabricationprocess to generate the IC, which may involve a multiple-step sequenceof photo lithographic and chemical processing steps during whichelectronic circuits are gradually created on a wafer made ofsemiconducting material. The circuit layout definition may be in theform of a mask which can be used in a lithographic process forgenerating an IC according to the circuit definition. Alternatively, thecircuit layout definition provided to the IC generation system 506 maybe in the form of computer-readable code which the IC generation system506 can use to form a suitable mask for use in generating an IC.

The different processes performed by the IC manufacturing system 502 maybe implemented all in one location, e.g. by one party. Alternatively,the IC manufacturing system 502 may be a distributed system such thatsome of the processes may be performed at different locations, and maybe performed by different parties. For example, some of the stages of:(i) synthesising RTL code representing the IC definition dataset to forma gate level representation of a circuit to be generated, (ii)generating a circuit layout based on the gate level representation,(iii) forming a mask in accordance with the circuit layout, and (iv)fabricating an integrated circuit using the mask, may be performed indifferent locations and/or by different parties.

In other examples, processing of the integrated circuit definitiondataset at an integrated circuit manufacturing system may configure thesystem to manufacture an out-of-bounds recovery circuit without the ICdefinition dataset being processed so as to determine a circuit layout.For instance, an integrated circuit definition dataset may define theconfiguration of a reconfigurable processor, such as an FPGA, and theprocessing of that dataset may configure an IC manufacturing system togenerate a an out-of-bounds recovery circuit having that definedconfiguration (e.g. by loading configuration data to the FPGA).

In some embodiments, an integrated circuit manufacturing definitiondataset, when processed in an integrated circuit manufacturing system,may cause an integrated circuit manufacturing system to generate adevice as described herein. For example, the configuration of anintegrated circuit manufacturing system in the manner described abovewith respect to FIG. 5 by an integrated circuit manufacturing definitiondataset may cause a device as described herein to be manufactured.

In some examples, an integrated circuit definition dataset could includesoftware which runs on hardware defined at the dataset or in combinationwith hardware defined at the dataset. In the example shown in FIG. 5,the IC generation system may further be configured by an integratedcircuit definition dataset to, on manufacturing an integrated circuit,load firmware onto that integrated circuit in accordance with programcode defined at the integrated circuit definition dataset or otherwiseprovide program code with the integrated circuit for use with theintegrated circuit.

Those skilled in the art will realize that storage devices utilized tostore program instructions can be distributed across a network. Forexample, a remote computer may store an example of the process describedas software. A local or terminal computer may access the remote computerand download a part or all of the software to run the program.Alternatively, the local computer may download pieces of the software asneeded, or execute some software instructions at the local terminal andsome at the remote computer (or computer network). Those skilled in theart will also realize that by utilizing conventional techniques known tothose skilled in the art that all, or a portion of the softwareinstructions may be carried out by a dedicated circuit, such as a DSP,programmable logic array, or the like.

The methods described herein may be performed by a computer configuredwith software in machine readable form stored on a tangible storagemedium e.g. in the form of a computer program comprising computerreadable program code for configuring a computer to perform theconstituent portions of described methods or in the form of a computerprogram comprising computer program code means adapted to perform allthe steps of any of the methods described herein when the program is runon a computer and where the computer program may be embodied on acomputer readable storage medium. Examples of tangible (ornon-transitory) storage media include disks, thumb drives, memory cardsetc. and do not include propagated signals. The software can be suitablefor execution on a parallel processor or a serial processor such thatthe method steps may be carried out in any suitable order, orsimultaneously.

The hardware components described herein may be generated by anon-transitory computer readable storage medium having encoded thereoncomputer readable program code.

Memories storing machine executable data for use in implementingdisclosed aspects can be non-transitory media. Non-transitory media canbe volatile or non-volatile. Examples of volatile non-transitory mediainclude semiconductor-based memory, such as SRAM or DRAM. Examples oftechnologies that can be used to implement non-volatile memory includeoptical and magnetic memory technologies, flash memory, phase changememory, resistive RAM.

A particular reference to “logic” refers to structure that performs afunction or functions. An example of logic includes circuitry that isarranged to perform those function(s). For example, such circuitry mayinclude transistors and/or other hardware elements available in amanufacturing process. Such transistors and/or other elements may beused to form circuitry or structures that implement and/or containmemory, such as registers, flip flops, or latches, logical operators,such as Boolean operations, mathematical operators, such as adders,multipliers, or shifters, and interconnect, by way of example. Suchelements may be provided as custom circuits or standard cell libraries,macros, or at other levels of abstraction. Such elements may beinterconnected in a specific arrangement. Logic may include circuitrythat is fixed function and circuitry can be programmed to perform afunction or functions; such programming may be provided from a firmwareor software update or control mechanism. Logic identified to perform onefunction may also include logic that implements a constituent functionor sub-process. In an example, hardware logic has circuitry thatimplements a fixed function operation, or operations, state machine orprocess.

Any range or device value given herein may be extended or alteredwithout losing the effect sought, as will be apparent to the skilledperson.

It will be understood that the benefits and advantages described abovemay relate to one embodiment or may relate to several embodiments. Theembodiments are not limited to those that solve any or all of the statedproblems or those that have any or all of the stated benefits andadvantages.

Any reference to ‘an’ item refers to one or more of those items. Theterm ‘comprising’ is used herein to mean including the method blocks orelements identified, but that such blocks or elements do not comprise anexclusive list and an apparatus may contain additional blocks orelements and a method may contain additional operations or elements.Furthermore, the blocks, elements and operations are themselves notimpliedly closed.

The steps of the methods described herein may be carried out in anysuitable order, or simultaneously where appropriate. The arrows betweenboxes in the figures show one example sequence of method steps but arenot intended to exclude other sequences or the performance of multiplesteps in parallel. Additionally, individual blocks may be deleted fromany of the methods without departing from the spirit and scope of thesubject matter described herein. Aspects of any of the examplesdescribed above may be combined with aspects of any of the otherexamples described to form further examples without losing the effectsought. Where elements of the figures are shown connected by arrows, itwill be appreciated that these arrows show just one example flow ofcommunications (including data and control messages) between elements.The flow between elements may be in either direction or in bothdirections.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein. In view of the foregoing description itwill be evident to a person skilled in the art that variousmodifications may be made within the scope of the invention.

What is claimed is:
 1. An out-of-bounds recovery circuit for anelectronic device, the out-of-bounds recovery circuit comprising:detection logic configured to: monitor one or more control and/or datasignals of the electronic device; and detect an out-of-bounds violationin the electronic device, when the detection logic determines, based onthe one or more control and/or data signals of the electronic device, aprocessing element of the electronic device has fetched an instructionfrom a non-allowable memory address for a current operating state of theelectronic device; and transition logic configured to, in response tothe detection logic detecting an out-of-bounds violation, cause theelectronic device to transition to a predetermined safe state andinvalidate the fetched instruction so that the processing element doesnot execute the instruction fetched from the non-allowable memoryaddress.
 2. The out-of-bounds recovery circuit of claim 1, wherein theone or more control and/or data signals includes a signal indicating acurrent value of a program counter of the processing element, and thedetection logic is configured to determine the processing element hasfetched an instruction from a non-allowable memory address for thecurrent operating state of the electronic device when the current valueof the program counter does not fall within at least one allowablememory address range for the current operating state of the electronicdevice.
 3. The out-of-bounds recovery circuit of claim 1, wherein thecurrent operating state is a boot operating state in which theelectronic device executes boot firmware.
 4. The out-of-bounds recoverycircuit of claim 1, wherein the electronic device is operable in one ormore operating states and each operating state is associated with one ormore allowable memory address ranges.
 5. The out-of-bounds recoverycircuit of claim 4, wherein the detection logic comprises address rangegenerator logic configured to identify the one or more allowable memoryaddress ranges associated with each of the one or more operating states.6. The out-of-bounds recovery circuit of claim 4, wherein the one ormore control and/or data signals includes one or more signals indicatingthe current operating state of the electronic device.
 7. Theout-of-bounds recovery circuit of claim 6, wherein the electronic deviceis operable in a boot operating state and a normal operating state, andthe one or more signals indicating the current operating state of theelectronic device comprises a signal indicating whether a power onsequence is complete.
 8. The out-of-bounds recovery circuit of claim 1,wherein the detection logic comprises an out-of-bounds violationdetected register, and the detection logic is configured to, in responseto detecting an out-of-bounds violation, set the out-of-bounds violationdetected register; and the transition logic is configured to determinethat the detection logic has detected an out-of-bounds violation basedon the out-of-bounds violation detected register.
 9. The out-of-boundsrecovery circuit of claim 1, wherein the detection logic is configuredto, in response to detecting an out-of-bounds violation, generate asignal indicating that the detection logic has detected an out-of-boundsviolation, and the transition logic is configured to determine whetherthe detection logic has detected an out-of-bounds violation based on thegenerated signal.
 10. The out-of-bounds recovery circuit of claim 1,wherein the transition logic is configured to cause the electronicdevice to transition to the predetermined safe state by setting thestate of the processing element to a particular state.
 11. Theout-of-bounds recovery circuit of claim 10, wherein the particular stateis an idle state.
 12. The out-of-bounds recovery circuit of claim 1,wherein the transition logic is configured to cause the electronicdevice to transition to the predetermined safe state by invoking aninterrupt.
 13. The out-of-bounds recovery circuit of claim 1, whereinthe out-of-bounds recovery circuit is embodied in hardware on anintegrated circuit.
 14. A method of recovering an electronic device froman out-of-bounds violation, the method comprising: monitoring, by anout-of-bounds recovery circuit, one or more control and/or data signalsof the electronic device; detecting, by the out-of-bounds recoverycircuit, an out-of-bounds violation in the electronic device when it isdetermined, from the one or more control and/or data signals of theelectronic device, that a processing element of the electronic devicehas fetched an instruction from a non-allowable memory address for acurrent operating state of the electronic device; and in response todetecting an out-of-bounds violation: causing, by the out-of-boundsrecovery circuit, the electronic device to transition to a predeterminedsafe state; and invaliding the fetched instruction so that theprocessing element does not execute the instruction fetched from thenon-allowable memory address.
 15. The method of claim 14, wherein theone or more control and/or data signals includes a signal indicating acurrent value of a program counter of the processing element, and it isdetermined that the processing element has fetched an instruction from anon-allowable memory address for the current operating state of theelectronic device when the current value of the program counter does notfall within at least one allowable memory address range for the currentoperating state of the electronic device.
 16. The method of claim 14,wherein the electronic device is operable in one or more operatingstates and each operating state is associated with one or more allowablememory address ranges.
 17. The method of claim 14, wherein causing theelectronic device to transition to a predetermined safe state comprisessetting the state of the processing element to a particular state. 18.The method of claim 14, wherein causing the electronic device totransition to a predetermined safe state comprises invoking aninterrupt.
 19. A non-transitory computer readable storage medium havingstored thereon computer readable instructions that, when executed at acomputer system, cause the computer system to perform the method as setforth in claim
 14. 20. A non-transitory computer readable storage mediumhaving stored thereon a computer readable dataset description of anintegrated circuit that, when processed in an integrated circuitmanufacturing system, causes the integrated circuit manufacturing systemto manufacture the out-of-bounds recovery circuit as set forth in claim1.